The removal of sulfur dioxide from flue gases in a wet or liquid scrubbing system is well known and a commercially used system. Such scrubbing systems use alkaline earth metal components such as limestone, lime, or magnesium oxide or hydroxide. Preferably, the scrubbing slurry is formed from calcium hydroxide and magnesium hydroxide, such as, for example, is disclosed in U.S. Pat. Nos. 3,919,393, 3,919,394 and 3,914,378, all assigned to the assignee of the present invention.
Sulfite oxidation is a very important side reaction in flue gas desulfurization (FGD) processes using a lime or limestone aqueous scrubbing slurry. When the solid waste oxidation level is above 15%-20%, the solution will be saturated with gypsum. The gypsum saturated solution will cause some serious reliability problems such as scaling on scrubber internals and mist eliminators. Thiosulfate (S.sub.2 O.sub.3.sup.-2) has been indentified as a free radical scavenger which inhibits sulfite oxidation effectively, as discussed in "Thiosulfate as an Oxidation Inhibitor in Flue Gas Desulfurization Processes: A Review of R & D Results", G. T. Rochelle, et al., presented at the 9th Symposium on Flue Gas Desulfurization, Cincinnati, Ohio, Jun. 4-7, (1985). Today, it is a general practice to apply thiosulfate to wet scrubbers to enhance operating reliability. In addition to inhibiting sulfite oxidation, thiosulfate is also found to improve the dewatering characteristics of calcium sulfite/sulfate waste solids.
Currently, there are two lime slurry scrubbing power stations using sodium thiosulfate and at least six limestone slurry scrubbing power stations using emulsified sulfur, which reacts with sulfite to form thiosulfate, to reduce sulfite oxidation and in consequence to enhance operating reliability as discussed in "Results of Wet FGD Testing at EPRI's High-Sulfur Test Center", R. E. Moser, et al., presented at the First Combined FGD and Dry SO.sub.2 control Symposium, St. Louis, Mo. October (1988). Sulfur addition, if high conversion to thiosulfate can be achieved, is preferred because of economical reasons. A very long residence time is usually necessary for high sulfur conversion to thiosulfate because of the slow reaction between sulfur and sulfite/bisulfite.
Thiosulfate has been produced by the reaction of sulfur and sulfite in neutral or alkaline media according to the equation: EQU S+SO.sub.3.sup.-2 .fwdarw.S.sub.2 O.sub.3.sup.-2 ( 1)
Because of the equilibrium between sulfite and bisulfite: EQU HSO.sub.3.sup.- .fwdarw.H.sup.+ +SO.sub.3.sup.-2 ( 2)
alkalinity is necessary to enhance reaction (1) by removing the H.sup.+ released from bisulfite. In addition, thiosulfate will decompose to sulfur and bisulfite in acidic solution: EQU S.sub.2 O.sub.3.sup.-2 +H.sup.+ .fwdarw.S+HSO.sub.3.sup.- ( 3)
which is essentially the reverse reaction of equation (1).
Reaction (1) is very slow primarily because of the low solubility (and slow dissolution rate) of sulfur in aqueous solution. It was found that when pH is above 5.0 and when the concentration of sulfite/bisulfite is greater than 80 ppm SO.sub.3.sup.-2, the reaction rate is independent of pH or sulfite concentration and is first order in sulfur solids concentration as discussed in "The Reaction of Colloidal Sulfur With Sulfite", G. W. Donaldson and F. J. Johnson, J. Phys. Chem. Vol. 73, pp. 2064-2068 (1969). At 131.degree. F., a typical temperature at lime/limestone slurry scrubbers and recycle tanks, 0.04 hr.sup.-1 was suggested for this first order kinetic constant (Rochelle et al., "Thiosulfate Additives for Lime/Limestone Scrubbing", EPA-600/9-87-004b, February 1987).
One way to facilitate the production of thiosulfate is using polysulfides instead of sulfur: EQU S.sub.x.sup.-2 +SO.sub.3.sup.- .fwdarw.S.sub.(x-1).sup.-2 +S.sub.2 O.sub.3.sup.-2 ( 4)
Polysulfides may be formed by dissolving sulfur in sulfide solutions (E. S. Gould, "Inorganic Reactions and Structures", Revised Ed. pp. 291-292, 1962). Reaction 4 is so fast that proton released by reaction 2, in some instances, cannot be neutralized immediately. The consequence is the formation of a "local" low pH spot despite the alkaline bulk solution and the generation of hydrogen sulfide (H.sub.2 S) that will cause serious odor problems. Because of this reason, polysulfide was abandoned in the EPA limestone scrubbing pilot plant at Research Triangle Park (RTP) although it was effective in producing thiosulfate in situ (Rochelle et al., 1987). However, if polysulfides are introduced to scrubbers together with lime slurries, the high pH of the feeding slurry and the scrubbing liquor as well as the fast dissolution of lime will neutralize the proton in reaction 2 effectively and therefore prevent the formation of H.sub.2 S.
One logical approach to produce thiosulfate in situ, is the alkaline hydrolysis of sulfur: ##STR1##
The major reason why this method is not tried is because of the sad fact that the extreme conditions (high pH and high temperature) required for reaction 5 are just not the characteristics of limestone slurry scrubbing systems. Because of the highly exothermic reaction of lime slaking, the temperature at the slaker is around 180.degree. F. depending on the lime/water ratio, but the residence time is usually short. On the other hand, the temperature at the lime slurry storage tank is lower (100.degree.-120.degree. F.), but the residence time is substantially longer. A good conversion of sulfur to polysulfides-precursors of thiosulfate in slakers/lime slurry storage tanks, is achievable because of the characteristics of lime slurry scrubbing systems. This kind of conversion of sulfur to thiosulfate and its precursors-polysulfides, is therefore, the privilege of lime slurry scrubbing systems.